1. Field of the Invention
The present invention relates to a wireless device in which front-ends and a baseband unit are separately provided at different positions and are connected to each other by wire.
2. Description of the Related Art
In in-vehicle wireless devices, a wireless device (receiver) and an antenna are provided so as to be separated from each other. Therefore, in order to prevent signal loss or a mixture of noise, a thick coaxial cable needs to be provided, which makes it difficult to perform wiring. In addition, since the thick cable is used, the weight of the wireless device increases, and manufacturing costs also increase.
In order to solve these problems, a front-end unit may be provided immediately below the antenna. When the front-end unit of the wireless device is provided immediately below the antenna, signals can be transmitted to a baseband unit with low signal loss or at a low frequency (an intermediate frequency or a baseband frequency) capable of reducing the influence of noise. In addition, when digital transmission is performed between the front-end unit and the baseband unit, it is possible to further reduce connection loss or the influence of noise. The transmission of signals between the front-end unit and the baseband unit is performed as follows. The front-end unit converts a received analog signal into a digital signal, further converts the digital signal into a serial signal, and transmits the serial signal to the baseband unit. The baseband unit converts the received serial digital signal into a parallel digital signal and further converts the digital signal into an analog signal. In this way, it is possible to convert an analog signal including a control signal into a digital signal.
As such, when the front-end unit and the baseband unit are provided so as to be separated from each other, a unit for achieving clock synchronization between the front-end unit and the baseband unit is needed in order to achieve clock synchronization between an AD converter of the front-end unit and a baseband process. When a wiring line for a reference clock is provided for clock synchronization, the number of wiring lines is increased.
For example, the clocks of local oscillators or AD converters between the front-ends need to be synchronized with each other in order for a plurality of front-ends to receive signals from a plurality of antennas and to perform diversity combining between a plurality of signals (Japanese Patent Application Laid-Open (JP-A) No. 2001-345778). When a plurality of front-end units receives signals of different wireless systems and one baseband unit processes the received signals at the same time, it is necessary to perform clock synchronization between the baseband unit and each front-end unit. When a wiring line is provided for each reference clock for clock synchronization between the front-end unit and the baseband unit, the number of wiring lines is increased.
When data is transmitted in serial, the clock is superimposed on data, and the data is reproduced from the clock. In this way, it is possible to avoid the addition of wiring lines. However, this structure has the following problems. First, clock synchronization is not appropriately performed due to the degree of purity of signals during clock reproduction. Second, when data transmission is simultaneously performed using a plurality of wireless systems with different symbol rates, it is necessary to use a clock frequency common to a plurality of wireless systems in order to achieve the synchronous transmission of data, and a common minimum clock frequency may be excessively high according to combinations of the symbol rates. In order to solve the second problem, a sampling frequency conversion method has been proposed which performs arbitrary frequency conversion into a predetermined common clock. However, signal distortion occurs in this conversion method.